40 rEK.SIJ)K-\'iIA]> ADDltKSS SKCTIOX 13. 



give a general idea of tliQ geology much more frequeiitly 

 tliaii is the case. Another jjoint to Avhich reference maj- be 

 made is the necessity of discrimination between various types 

 of association in dealing with the relations between ore-bodies 

 and rocks. This concerns the mining geologist as much as 

 the engineer, who does not profess to be a geological expert. 

 Thus, there is a world of difference between the occurrence 

 of an ore-body in a rock and the mere presence in the vicinity 

 of some rock assumed to have a genetic connection with the 

 deposition of the ore. 



This leads me into what is more especially the province 

 of the mining- geologist. In discussing the origin of ore- 

 bodies, it is clear that there are two perfectly distinct types 

 of genetic association. These are well illustrated by the ores 

 which are usually found in close association with acid and 

 basic igneous rocks respectively. There are, it is true, a 

 number of very important metals — gold, silver, copper, lead, 

 zinc, antimony, etc. — which appear to have no very well- 

 marked tendencj', excejDt over limited areas, to occur in 

 connection with one particular rock type rather than with 

 others. But with other minerals the case is different. There 

 are, in fact, two very well-defined groups of minerals, one of 

 which occurs almost exclusively in conjunction with basic 

 rocks like serpentine, while the other is almost invariably 

 associated with acid rocks like granite and its offshoots. The 

 basic group includes tlie metals platinum, chromium, 

 manganese, nickel and cobalt, as well as magnesite, talc, 

 asbestos, and also the diamond. The acid grouj^ comprises 

 such metals as tin, tungsten, bismuth, molybdenum, uranium, 

 tantalum, zirconium, cerium and thorium, together with 

 minerals like mica and, perhaps, graphite. Now, a little con- 

 sideration will show that some of the leading members of 

 these two groups differ radically in the manner of their 

 association with the rocks which appear to control their 

 occurrence. The minerals of the basic group are almost 

 invariably found inside the limits of the rock mass with which 

 we associate them. Platinum and chromite, for example, are 

 found in such rocks as serpentine and its allies, and these 

 rocks form both the matrix, or lodestuff', and the country of 

 the deposit. With some of the minerals of the acid group 

 the case is ciuite different. Tin and tungsten, for instance, 

 and notably the richest and most persistent deposits of those 

 metals, do' not occur as a rule in a granite matrix, or even 

 with granite as their country rock. They generally favour 

 a ciuartz-ore matrix, which seems to be connected with granitic 

 intrusions, as shown by the invariable presence of granite 

 in the near vicinity, but they frequently have sedimentaij 

 or metamorphic rocks forming the walls of the lode. This 

 obviously indicates a difference in the nature of their genetic 

 connection with the associated rock types, but, as a discussion 

 of the matter is likely to lead us into regions where con- 

 troversy is still acute,! do not propose to go further into it 

 here. 



